The present invention relates to a developer composition for presensitized plates for use in making lithographic printing plates (hereunder referred to as "developer composition for PS plate") and a plate-making method in which such a developer composition is employed and more specifically to a developer composition for removing portions (which serving as non-image areas) of negative-working and/or positive-working PS plates and a plate-making method in which the developer composition is employed.
Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J.P. KOKOKU") No. Sho 56-39464 discloses a developer composition for removing non-image areas of a light-sensitive layer which comprises a weak alkaline aqueous solution-soluble organic high molecular weight polymer and a light-sensitive diazo resin. However, this developer composition does not always show ability of completely dissolving even the light-sensitive layer in non-exposed areas adjacent the surface of a substrate. For this reason, background contamination is correspondingly caused when printing operation is performed using a lithographic printing plate obtained by developing a PS plate with this developer composition.
It is known that a drawback of this kind can be eliminated by the addition of a sulfite such as those disclosed in J.P. KOKOKU No. Sho 56-42860 (U.S. Pat. No. 4,186,006) to the foregoing developer composition. The problem of such background contamination of non-image areas during printing can surely be eliminated by the addition of a reducing inorganic sulfite, but if the developer is stored for a long time period, a part of the sulfite is oxidized. This leads to the reduction in the contamination-inhibitory effect of the developer.
On the other hand, as a developer containing hard water is deteriorated, a binder is separated out from the developer, which possibly results in the formation of sludge. On adding a chelating agent (a hard water softener) to such a developer for preventing the formation of sludge, the oxidation of the sulfite is accelerated.
On the other hand, a PS plate carrying a positive-working light-sensitive layer has also been widely used like PS plates provided with negative-working light-sensitive layers. However, the composition of developers for the positive-working PS plate in general differs from that of developers for the negative-working ones. Therefore, a plurality of developing machines are required for treating both positive- and negative-working PS plates. This leads to an increase in the operating cost. In addition, the space for installing the developing machines is too great and the operating efficiency is also insufficient.